Minimally invasive devices are often employed for medical procedures, including those involving ablation, dilation, and the like. In a particular situation, an ablation procedure on may involve creating a series of inter-connecting or otherwise contiguous lesions in order to electrically isolate tissue believed to be the source of an arrhythmia. Such lesions may be created using a variety of different energy transmission modalities, such as cryogenic freezing or heating with radiofrequency (“RF”) energy, for example.
Radiofrequency or other ablation devices often include one or more electrically conductive surfaces or electrodes to impart electrical or thermal energy conduction through a tissue site. During operation, the tissue heats up, thus heating the electrodes that are in tissue contact. When positioned in anatomical areas that have high fluid (blood, etc.) flow, a portion of the electrodes are cooled via convection with the passing fluid, thus dissipating heat as tissue is ablated. The portions of the electrodes engaged with the tissue are typically not exposed to the surrounding fluid flow, and as a result, are not cooled to the same degree. Indeed, the contacted surface of the electrodes are heated through conduction by the tissue itself as it is ablated, and a temperature sensor in contact with this section of the electrode or tissue could provide an accurate tissue/electrode interface temperature. This ability to accurately measure the tissue/electrode interface temperature can be used as a means of feedback to control the energy being applied through the electrodes and to the tissue. Exceeding a particular temperature range or threshold can result in unwanted injury to the tissue site, including tissue charring, and can also compromise the medical device itself.
When an ablation device is located in an area of little or no physiological fluid flow, the ability for the electrode to dissipate heat is hampered, which limits the delivery of ablation energy modulated by a measured temperature response, e.g., the temperature rises more quickly, which may require a quicker reduction in the powering of the device and thus, reduced treatment efficacy. Current ablation systems that deal in this temperature-response regime sometimes rely on other measured responses (e.g. impedance) as an alternate means of control loop feedback, or can operate in a feed-forward control mode (e.g. set the ablation energy source to a fixed power setting). However, by not relying on the tissue/electrode interface temperature, the latter two methods of treatment control can again invite the undesired injury resulting from excess heat generated by the tissue.
To supplement cooling a device operating in an area having little physiological fluid flow, the delivery of an irrigation or cooling fluid is sometimes utilized (e.g., irrigated radio frequency ablation catheters and similar actively cooled devices). The design intent of active cooling is to allow the ablation electrode(s) to dissipate heat in low or no flow areas of the heart or other anatomy by flooding or circulating the electrode with saline or other cooling fluid. Unfortunately, such irrigated devices and methods cannot rely on dynamic temperature measurement as a means of feedback control because when the ablation electrode or surface is cooled directly, a temperature sensor in contact with ablation electrode is also subjected to the cooling fluid, which compromises its ability to accurately measure the actual temperature at the electrode/tissue interface. The compromised accuracy of the temperature measurements again invites injury from excessive heat buildup.
In view of the above, it is desirable to provide effective cooling mechanisms for medical devices used in environments having low fluid flow rates or reduced ambient cooling without compromising the ability to accurately measure or monitor temperature of a device-tissue interface for control or operation of the device.